Method of making refractory metal nitride fibers, flakes and foil



Feb. 27, 1968 R. L HOUGH 3,370,923

METHOD OF MAKING REFRACTORY METAL NITRIDE mamas, FLAKES AND FOIL FiledSept. 15, 1964 INVENTOR l ma /v A. #00619 United States Patent 3,370,923METHOD OF MAKING REFRACTORY METAL NITRIDE FIBERS, FLAKES AND FOIL RalphL. Hough, Springfield, Ohio, assignor to the United States of America asrepresented by the Secretary of the Air Force Filed Sept. 15, 1964, Ser.No. 396,780 Claims. (Cl. 23-191) ABSTRACT OF THE DISCLOSURE Method formaking thin section nitride fibers, flakes or foil of a clean refractorymetal selected from Nb, Ta, Ti, Hf and Zr by rolling the metal to athinness of from to 10 inch, positioning the metal on a conveyor beltchemically inert to the metal prior to the passage of the belt betweenan upper and a lower bathe of gas entering an oxygen-free nitridingchamber of a furnace with an atmosphere of nitrogen or ammonia whereinthe bafiies direct the gas flow against both the top and the bottom ofthe metal on the conveyor belt, and maintaining the temperature of themetal within the range of the nitriding temperature of the meal on theconveyor belt.

The invention that is described herein may be manufactured and used byor for the United States Government for governmental purposes withoutthe payment to me of any royalty thereon.

' This invention concerns a new and improved process for the nitridingof fibers, flakes and filaments of the refractory metals Nb(Cb), Ta, Ti,Hf, and Zr and to a furnace suitable for practicing the process. Thematerial is used as missiles, nose cones, aircraft wing leading edgesand the like.

The previous synthesis of metal nitrides has commonly been as coatingson substrates that are accomplished by reacting a volatile metal halidewith hydrogen and with nitrogen or with ammonia, at a temperature ofabout between 2000 to 4000 F. by vapor deposition as a thin coating onthe substrate.

The object of the present invention is to provide a new and improvedmethod for making thin section nitride fibers, filaments, flake andfoil, of a clean refractory metal. The fibers etc. are impregnated witha plastic in making reinforcement for structural and ablative materialthat reiaiTlS its identity and its mechanical strength under greatstresses and under high shear forces at high temperatures in the orderof 5000 F. or 2760 C.

An illustrative embodiment of a suitable furnace arrangement in whichthe present invention is practiced is represented in the accompanyingdrawing wherein:

FIG. 1 is a side elevational view partly broken away and in section of arectangular tubular furnace wherein the present invention may bepracticed;

FIG. 2 is a diminished section taken along the line 22 of FIG. 1; and

FIG. 3 is a fragmentary plan view of the metal foil undergoing thermaldiffusion of oxygen and nitrogen by temperature gradients.

In FIG. 1 the furnace wall 10 is of rectangular cross section and hasrectangular ends 11 and 11. Siits 12 in the ends 11 and 11 of thefurnace 10 and dimensioned for a minimum clearance for the passagethrough the slits of a silica cloth conveyor belt 13 carrying strips offoil 14 the length of the furnace 10. Adjacent to the foil input end ofthe furnace 10 are a pair of baffles 15 and 15' that are positioned onopposite sides of the belt 13 and foil 14. The bafiles 15 and 15illustratively are cantilever supported from a pair of water conductingcooling pipes 16 and 16 that are coaxial with a nitrogen conducting pipe17.

The thermal diffusion of oxygen and nitrogen in zirconium is discussedby G. D. Ruick and H. A. C. M. Bruning in volume 190, Nature, p. 1181,published in 1961.

In FIG. 3 of the accompanying drawing is indicated the fragment of thefoil 14 at the input end of the furnace 10 and against a proximal edgeof which nitrogen gas from the pipe 17 is discharged into the furnace10.

In FIG. 3, T is the nitriding temperature within the furnace 10 and ishigher than the temperature T of the continuously moving foil 14, as itsedge passes through the nitrogen gas that is supplied from the pipe 17to the furnace 10. The nitrogen gas within the furnace 10 is held at apressure that is sufficiently above ambient to maintain continuouslypure nitrogen within the furnace 10.

The foil temperature T is sufliciently less than T such that thenitriding of the foil does not occur appreciably at T and such that inthe zone A there exists a thermal gradient that causes oxygen occludedin the metal of the foil 14, to migrate by diffusion to the cooler edge20 of the foil 14 where it appears as a dark discoloration in the endproduct.

The foil 14 may be replaced by short wires that extend across the belt13. The gas diffusion causes a dark discoloration adjacent the ends ofthe wire A corresponding diffusion of gaseous impurities discolors theends of foil or Wire that is adjacent a metal support for the foil orwire.

In the described manner traces of oxygen can be eliminated from the foilor wire by thermal diffusion of the Ludwig-Soret type. This phenomenonis established in this process by the thermal gradient within thereaction zone.

The oxygen diffuses to the zone of lower temperature across the thermalgradient.

Thermal diffusion is the moving of atoms of impurities, here oxygen,within a metal under the influence of a temperature gradient, with theoxygen atoms moving toward lower temperatures.

The nitriding of ultra fine wires of pure zirconium meta-l in the orderof 0.001 inch diameter at the temperature of from 1600 F. (871 C.) to2600 (1427 C.) is most difficult to accomplish with pure nitrogen orwith ammonia, because of the reaction kinetics that are involved. Thenitriding time of these ultra fine Zr wires of 0.001 inch diameterwithin the temperature range of from 1600 F. to 2600 F., is up to aboutone minute.

The reaction kinetics of nitriding zirconium are such that it is themost time consuming of the group of refractory metals, that consists ofniobium, tantalum, titanium, hafnium, and zirconium. These metals arerolled to very thin foil or drawn as wire in the order of 0.001 inchthickness. The advantage of the fiat rolled wire is that its edgestrains are minimized by having an elliptical section or an ellipticallycontoured edge.

The metal thin foil or fiat rolled wire is placed in a scavenged andevacuated chamber maintained in the temperature range of from 1800 to2700 F. (982 to 1482 C.). The chamber may be a mufile furnace for batchproduction, or a fused quartz tubular furnace with a conveyor beltpassing axially through the tube for continuous production. The furnacepreferably is initially scavenged or baked out and then is evacuated.During the production of the nitrided metal, the furnace is suppled withan atmosphere of ultra pure nitrogen or ammonia at slightly aboveatmospheric pressures. The group of refractory metals may -beillustrated by Zirconium.

The nitrogen or the ammonia is procured as cylinder grade gas.Illustratively, the nitrogen is passed through a dessicant, such as adesired succession of increasingly dehydrated containers of magnesiumperchlorate, for the drated nitrogen is cleaned of oxygen, as by beingpassed through a desired plurality of beds of copper turnings that aremaintained about in the range of from 1400 to 1800 F. (760 to 982 C.).The resultant nitriding gas is supplied continuously as the ultra pureatmosphere in the chosen nitriding chamber.

The ultra pure nitriding gas in the chamber is passed over the zirconiumor other foil, or flat rolled wire at 1800 to 2700" F., at a fiow ratethat is maintained at the rate of the complete nitriding of the selectedrefractory metal end product. The use of the long tube furnace with aconveyor system passing longitudinally through the tube requires gasbaffiing to prevent back difiusion of ambient gases into the reactionzone, or requires the maintaining of a closed system for the entireoperation.

The nitriding process is applied to foil or wire that is about in therange of from 0.001 to 0.0001 inch thick. With the refractory metal0.00014 inch Zr foil at 1920 F., the Zr is completely nitrided in oneminute, by experimental determination. The ZrN so made had a meltingpoint of 2980 C. or 5400" F. without sublimation or decomposition undera pressure of one atmosphere; an X-ray density of 7.349 g./cm. a vaporpressure below at 1730 C.; and a hardness of 8-9 mohs. The nitrides ofNb, Ta, Ti, and Zr are substantially chemically inert at roomtemperature of 22 C., being attacked only by mixtures of strong acidsand oxidizing agents.

The resultant ZrN in thickness in the order of 10 inch thick was thenused by being combined with a polymerize d plastic such illustrativelyas an epoxy resin or the like, as a reinforced structural and ablativeplastic composite for use at very high temperatures in the order of 2950to 3310 C., with the exception of niobium nitride which melts at 2050C., and of excellent thermal shock resistance. The fibrous material ofzirconium nitride packed resin makes a tough and serviceable materialfrom which are made ablative plastics to serve as the lead edges ofaircraft wings and the like, by a molding operation.

It is to be understood that the details of the process that aredisclosed herein may be modified somewhat without departing from thespirit and the scope of the present invention- H I claim:

1. The method of making refractory metal fibrous nitride flake byrolling a refractory metal selected from the group that consists of Nb,Ta; Ti, Hf and Zr to a thickness about in the range of from 10- to 10-inch; positioning the rolled refractory metal on top of a conveyor beltthat is chemically inert to the metal, passing the metal on'the' beltbetween an upper and a lower bafile of gas entering an oxygen-freenitriding chamber of a furnace with an atmosphere selected from thegroup of gaseous nitrogen and ammonia at a pressure above ambientwherein the pair of bafiles direct the gas flow against both the top andthe bottom of the refractory metal on the conveyor belt, maintaining thetemperature of the refractory metal about within the range of thenitr'iding temperature of the metal on the conveyor belt, and removingthe nitrided refractory metal from the nitriding chamber of the furnace.

2. The method described in claim 1 wherein the conveyor belt is made ofsilica cloth.

3. The method defined by claim 1 wherein the refractory metal is a foil.

4. The method defined by claim 1 wherein the refractory metal is a wire.

5. The method defined by claim 1 wherein the baflles extendsubstantially parallel to the direction of flow of a stream of gasentering the chamber and on opposite sides of the metal on the conveyorbelt.

References Cited UNITED STATES PATENTS 2,461,019 2/1949 Alexander 2319l2,750,268 6/1956 Erasmus et al 23-191 2,784,639 3/1957 Keenan et a1148-166 OSCAR R. VERTIZ, Primary Examiner.

H. S. MILLER, Assistant Examiner.

